Using a top-contact window, we have observed emission from a direct-gap monitor layer placed at the interface between the p-cladding and contact layers of an AlGaInP laser diode when driven under forward bias, thereby providing direct evidence for minority carrier (electron) leakage in these devices. We have further shown that the leakage is due to both drift and diffusion and, using pulsed optical excitation of a device under bias, we have determined a value of 170/spl plusmn/10 cm/sup 2/ V/sup -1/ s/sup -1/ for the mobility of minority carriers in the p-type cladding layer by a time-of-flight experiment. The data was analyzed using a simulation which takes account of the influence of recombination times in the well and monitor layer on the overall time response of the structure. The measured mobility corresponds to electron transport through the X-conduction band. We show that the drift component of the leakage current reduces the differential efficiency and is responsible for the decrease in external differential efficiency with increasing temperature. Because the leakage occurs by a mixture of drift and diffusion, the transit time does not decrease significantly with increasing drive current; however the impact of leakage on the modulation response is predicted to be very small unless the leakage becomes a substantial fraction of the total current.